Method for optimising liquefaction of natural gas

ABSTRACT

A method for liquefying a hydrocarbon stream such as natural gas starting from a feed stream.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a 371 of International Application PCT/FR2016/052024filed Aug. 3, 2016, which claims priority to French Patent Application1560731 filed Nov. 10, 2015, the entire contents of which areincorporated herein by reference.

BACKGROUND

The present invention relates to a method for liquefying a hydrocarbonstream such as natural gas in particular in a method for producingliquefied natural gas. At typical plants for liquefaction of natural gasusing a mixed refrigerant cascade, refrigerant streams are used forproducing cold at different levels of a main heat exchanger byevaporating against the hydrocarbon stream to be liquefied (typicallynatural gas).

Liquefaction of natural gas is desirable for a number of reasons. Forexample, natural gas can be stored and transported over great distancesmore easily in the liquid state than in gaseous form, as it occupies asmaller volume for a given mass and does not need to be stored at highpressure.

Several methods of liquefaction of a natural gas stream for obtainingliquefied natural gas (LNG) are known. Typically the mixed refrigerantis compressed by means of a compressor and separated into a gas streamand at least one liquid stream, then the two streams are combined toform a two-phase stream. This two-phase stream is fed into the main heatexchanger, where it is liquefied completely and subcooled to the coldesttemperature of the process, typically that of the stream of liquefiednatural gas. At the coldest outlet of the main heat exchanger, therefrigerant is expanded and fed back into the main exchanger, to beevaporated against the hydrocarbon-rich fraction that is beingliquefied.

This solution is not optimized, owing to the two-phase composition ofthe refrigerant stream once the two phases are recombined and introducedin this state into the exchanger. In fact the liquid refrigerant streamcontains the heaviest compounds. The latter will therefore evaporate ata higher temperature than lighter compounds such as nitrogen or methane,for example. It is therefore used for producing cold at an intermediatetemperature (typically of the order of −30° C. to −50° C., forprecooling and partial liquefaction of the hydrocarbon mixture to beliquefied).

As the gaseous refrigerant stream contains the lightest compounds, it isused for producing cold at a colder temperature (typically below −100°C.), for liquefaction and total subcooling of the hydrocarbon mixture tobe liquefied.

Therefore it is not necessary for the liquid refrigerant to be subcooledas much as the gaseous refrigerant before being expanded and evaporatedagainst the hydrocarbon stream to be liquefied. Now, this is what thetypical method of the prior art proposes, as described in the precedingparagraph.

Moreover, patent application US2009/0260392 A1 describes theliquefaction of a hydrocarbon-rich fraction against a mixed refrigerant,this refrigerant stream being separated in a phase separator into a gasphase and a liquid phase following a step of compression and cooling ofsaid mixed refrigerant. Next, the two phases of the refrigerant arecooled separately and then recombined only after the two phases havebeen expanded. Once recombined, these two phases are fed into theexchanger again in the form of a two-phase stream and heated against thenatural gas that is being liquefied. This “heating” occurs, both for theliquid phase of the refrigerant and for the gas phase, once thesestreams of the refrigerant have been expanded.

The inventors of the present invention then developed a solution forsolving the problem described above while optimizing the energyexpenditure.

SUMMARY

The solution proposed is to present the liquid refrigerant stream andthe gaseous refrigerant stream separately in the main heat exchanger.The liquid is then cooled to an intermediate temperature level, whereasthe gas is liquefied and cooled as far as the coldest outlet of the mainheat exchanger. The liquefied gaseous refrigerant is then expanded andfed back into the main heat exchanger. It is mixed with the cooledliquid refrigerant and also expanded beforehand, once it has reached thecorrect temperature level.

The present invention relates to a method for liquefying a hydrocarbonstream such as natural gas starting from a feed stream, comprising atleast the following steps:

Step a): passing the feed gas against a mixed refrigerant stream througha heat exchanger to supply an at least partially liquefied hydrocarbonstream having a temperature below −140° C.;

Step b): withdrawing a mixed refrigerant stream from the heat exchangerfrom an outlet where the temperature in the heat exchanger is highest;

Step c): feeding the mixed refrigerant from step b) into a phaseseparating means in order to produce a gaseous refrigerant stream and afirst liquid refrigerant stream;

Step d): passing the first liquid refrigerant stream resulting from stepc) into the heat exchanger starting from a first inlet and up to aso-called intermediate outlet, beyond which the refrigerant stream thusobtained is expanded, the temperature T1 at said outlet being such thatsaid expansion produces a gas fraction below 20%, preferably below 10%;

Step e): in parallel with step d), compressing the gaseous refrigerantstream resulting from step c) and then cooling before feeding therefrigerant stream thus obtained into a phase separating means in orderto produce a gaseous refrigerant stream and a second liquid refrigerantstream;

Step f): passing the second liquid refrigerant stream resulting fromstep e) through the heat exchanger starting from a second inlet and upto an outlet, beyond which the refrigerant stream thus obtained isexpanded, the temperature T2 at said outlet being above T1 and such thatsaid expansion produces a gas fraction below 20%, preferably below 10%;

Step g): passing the gaseous refrigerant stream resulting from step e)through the heat exchanger starting from a third inlet and up to anoutlet at a temperature T3, the level of which is the lowest of thetemperature levels of said heat exchanger in order to produce aliquefied stream, and then expansion of the stream thus obtained;

Step h): passing the stream resulting from step g) through the heatexchanger from an inlet at a temperature T3 to an outlet at atemperature approximately equal to the temperature T2;

Step i): mixing the refrigerant stream resulting from step h) with therefrigerant stream resulting from step f), and then passing the mixturethus obtained through the heat exchanger from an inlet having atemperature approximately equal to T2 to an outlet having a temperatureapproximately equal to T1;

Step j): mixing the refrigerant stream resulting from step i) with therefrigerant stream resulting from step d) and then passing the mixturethus obtained through the heat exchanger up to the outlet.

More particularly, the present invention relates to:

-   -   A method as defined above, characterized in that the mixed        refrigerant stream circulates in a closed-cycle refrigeration        circuit.    -   A method as defined above, characterized in that it comprises a        step before step c) of compressing the mixed refrigerant        resulting from step b) followed by cooling.    -   A method as defined above, characterized in that T1 is between        −30° C. and −50° C.    -   A method as defined above, characterized in that T2 is between        −80° C. and −110° C.    -   A method as defined above, characterized in that T3 is between        −140° C. and −170° C.    -   A method as defined above, characterized in that the mixed        refrigerant stream contains constituents among nitrogen,        methane, ethylene, ethane, butane and pentane.    -   A method as defined above, characterized in that the gaseous        refrigerant stream resulting from step e) contains nitrogen and        methane.    -   A method as defined above, characterized in that a pump is not        used.

The method according to the present invention makes it possible tooptimize the use of the liquid and gaseous refrigerant streams in theliquefaction cycle, since the liquid, which contains the heaviestcomponents, must not be subcooled as much as the gaseous refrigerant.

Moreover, a pump is not used in the method according to the invention,as the intermediate liquid (in the foregoing called the first liquidrefrigerant stream resulting from step c)) is not pumped in order to bemixed with the liquid at high pressure (in the foregoing called thesecond liquid refrigerant stream resulting from step e).

This is notably advantageous in terms of capital expenditure.

Although the method according to the present invention is applicable tovarious hydrocarbon feed streams, it is particularly suitable forstreams of natural gas to be liquefied. Furthermore, a person skilled inthe art will easily understand that, after liquefaction, the liquefiednatural gas may be further processed, if desired. As an example, theliquefied natural gas obtained may be depressurized by means of aJoule-Thomson valve or by means of a turbine. Furthermore, otherintermediate processing steps between gas/liquid separation and coolingmay be carried out. The hydrocarbon stream to be liquefied is generallya stream of natural gas obtained from natural gas or oil reservoirs.Alternatively, the stream of natural gas may also be obtained fromanother source, including a synthetic source such as a Fischer-Tropschprocess. Usually the stream of natural gas consists essentially ofmethane. Preferably, the feed stream comprises at least 60 mol % ofmethane, preferably at least 80 mol % of methane. Depending on thesource, the natural gas may contain amounts of hydrocarbons heavier thanmethane, such as ethane, propane, butane and pentane as well as certainaromatic hydrocarbons. The stream of natural gas may also containnon-hydrocarbon products such as H₂O, N₂, CO₂, H₂S and othersulfur-containing compounds, and others.

The feed stream containing natural gas may be pretreated before it isfed into the heat exchanger. This pretreatment may comprise reductionand/or removal of the undesirable components such as CO₂ and H₂S, orother steps such as precooling and/or pressurizing. Since these measuresare well known by a person skilled in the art, they are not described inmore detail here.

The expression “natural gas” as used in the present application refersto any composition containing hydrocarbons, including at least methane.This comprises a “crude” composition (before any treatment such ascleaning or washing), as well as any composition that has been treatedpartially, substantially or entirely for reduction and/or removal of oneor more compounds, including, but not limited to, sulfur, carbondioxide, water, and hydrocarbons having two or more carbon atoms.

The separator may be any unit, column or arrangement suitable forseparating the mixed refrigerant in a stream of refrigerant in vaporform and a stream of liquid refrigerant. Such separators are known inthe prior art and are not described in detail here.

The heat exchanger may be any column, a unit or other arrangementsuitable for allowing the passage of a certain number of streams, thusallowing direct or indirect heat exchange between one or more lines ofrefrigerant, and one or more feed streams.

BRIEF DESCRIPTION OF THE DRAWING

For a further understanding of the nature and objects for the presentinvention, reference should be made to the following detaileddescription, taken in conjunction with the accompanying drawing, inwhich like elements are given the same or analogous reference numbersand wherein:

The sole FIGURE illustrates one embodiment of the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

The invention will be described in more detail with reference to theFIGURE, which illustrates the scheme of a particular embodiment of animplementation of a method according to the invention.

In the FIGURE, a stream 1 of natural gas, optionally pretreatedbeforehand (typically having undergone separation of part of at leastone of the following constituents: water, CO₂, methanol,sulfur-containing compounds), is fed into a heat exchanger 2 in order tobe liquefied.

The FIGURE therefore shows a method for liquefying a feed stream 1. Thefeed stream 1 may be a pretreated stream of natural gas, in which one ormore substances, such as sulfur, carbon dioxide, and water, are reduced,so as to be compatible with cryogenic temperatures, as is known in theprior art.

Optionally, the feed stream 1 may have undergone one or more steps ofprecooling, as is known in the prior art. One or more of the precoolingsteps may comprise one or more refrigeration circuits. As an example, afeed stream of natural gas is generally treated starting from an initialtemperature of 30-50° C. Following one or more steps of precooling, thetemperature of the feed stream of natural gas may be reduced to −30 to−70° C.

In the FIGURE, the heat exchanger 2 is preferably a coil-wound cryogenicheat exchanger. Cryogenic heat exchangers are known in the prior art,and may have various arrangements of the feed stream(s) and refrigerantstreams. Furthermore, heat exchangers of this kind may also have one ormore lines to allow the passage of other streams, such as refrigerantstreams for other steps of a method of cooling, for example in methodsof liquefaction. These other lines or streams are not shown in theFIGURE, for simplicity.

The feed stream 1 enters the heat exchanger 2 via a feed inlet 3 andpasses through the heat exchanger via line 4, and then is withdrawn fromthe exchanger at outlet 5 to supply an at least partially liquefiedhydrocarbon stream 6. This liquefied stream 6 is preferably liquefiedcompletely and even subcooled, and may moreover be treated as discussedbelow. When the liquefied stream 6 is liquefied natural gas, thetemperature may be from about −150° C. to −160° C. Liquefaction of thefeed stream 1 is accomplished by means of a refrigerant circuit 7. Amixed refrigerant, preferably selected from the group comprisingnitrogen, methane, ethane, ethylene, propane, propylene, butane,pentane, etc., circulates in the refrigerant circuit 7. The compositionof the mixed refrigerant may vary according to the conditions and theparameters desired for the heat exchanger 2, as is known in the priorart.

In the arrangement of the operation of the heat exchanger 2 shown in theFIGURE, a gaseous refrigerant stream 8 is fed into the exchanger 2 at aninlet 9, then it passes through this inlet and is liquefied andsubcooled along line 10 through the heat exchanger 2, up to the outlet11. The temperature T3 of the outlet 11 is the lowest of thetemperatures of the heat exchanger 2. T3 is typically between −140° C.and −170° C., for example −160° C. During its passage through line 10,the stream of gaseous refrigerant 8 is liquefied, so that therefrigerant stream downstream of the outlet 11 is a liquid stream 12.The refrigerant stream 12 is then expanded for example by means of avalve 13, so as to supply a first refrigerant stream at reduced pressure14. This stream 14 is then fed into the heat exchanger 2 via the inlet15.

A liquid stream 16 of the refrigerant is fed into the heat exchanger 2via inlet 17, and then passes through the exchanger 2 along line 18. Theliquid stream of refrigerant 16 is withdrawn from the exchanger atoutlet 19, at an intermediate level between the top and the bottom ofsaid exchanger, having a temperature T2 above T3. For example, T2 isbetween −90° C. and −110° C. The refrigerant stream 20 downstream of theoutlet 19 is expanded in a pressure reducing valve 21, to form a secondstream of refrigerant at reduced pressure 22. The stream 22 then goes,via inlet 23, into the heat exchanger 2 again, and travels as far as theoutlet 24 of the heat exchanger.

Another liquid stream 25 of the refrigerant is fed into the heatexchanger 2 via inlet 26, and then passes through the exchanger 2 alongline 27. The liquid stream of refrigerant 25 is withdrawn from theexchanger at outlet 28, at an intermediate level between the top and thebottom of said exchanger, having a temperature T1 above T2. For example,T1 is between −30° C. and −50° C. The refrigerant stream 29 downstreamof the outlet 28 is expanded in a pressure reducing valve 30, to form athird stream of refrigerant at reduced pressure 31. Preferably, thepressures of the first, of the second and of the third refrigerant atreduced pressure 14, 22 and 31 are approximately the same; for exampleabout 3 bara.

Once it has entered the heat exchanger 2, the stream 14 of refrigerantevaporates, at least partially, up to the outlet 34, then downstream ofthis outlet 34 it will rejoin stream 22 resulting from expansion of thecooled liquid stream 16 of the refrigerant, and the two streams are thenmixed in stream 22. Similarly, this refrigerant stream 22 is mixed withrefrigerant stream 31 downstream of outlet 24.

Stream 31 then passes, via inlet 32, into the heat exchanger 2 again andevaporates completely up to the outlet 33 of the heat exchanger. Agaseous refrigerant stream 35 circulates in the refrigeration circuit 7downstream of the outlet 33 of the heat exchanger at ambient temperature(i.e. the temperature measured in the space where the device forimplementing the method according to the present invention is placed.This temperature is for example between −20° C. and 45° C.). Therefrigerant stream is compressed by a compressor 36. The method ofcompression is known from the prior art and the compressor 36 is forexample a compressor with at least two adiabatic sections A and B,therefore comprising at least two coolers 37 and 38. Once compressed inthe first section A of the compressor 36, the refrigerant stream 35 iscooled by means of a cooler 37 and is then partially condensed and formsa two-phase refrigerant stream 39. For example, the pressure at theoutlet of section A of the compressor 36 is of the order of 18 bara andthe temperature is of the order of 130° C. Typically the temperature atthe outlet of the cooler 37 is of the order of 25° C.

The refrigerant stream 39 is sent to a phase separator 40, whichseparates said two-phase refrigerant stream into a gas stream 41 and afirst liquid stream 25. Said first liquid refrigerant stream 25 consistsof the heaviest elements of the refrigerant stream of the refrigerationcircuit 7, i.e. in particular the components having more than fourcarbon atoms. The liquid refrigerant stream 25 then follows the pathdescribed above starting from the inlet 26 of heat exchanger 2.

The gaseous refrigerant stream 41 is compressed in section B of thecompressor. Typically, the pressure at the outlet of this section B isof the order of 50 bara. After this compression, the refrigerant streamis partially condensed by means of the cooler 38 and forms a two-phaserefrigerant stream 42. Typically the temperature is at the level of theambient temperature. The refrigerant stream 42 is sent to a phaseseparator 43, which separates said refrigerant stream into a gas stream8 and a second liquid stream 16. Said second liquid refrigerant stream16 consists of the elements that are lighter than those contained in theliquid 25 but heavier than those contained in the gas stream 8. Thisliquid refrigerant stream 16 then follows the path described abovestarting from the inlet 17 of heat exchanger 2. The gaseous refrigerantstream 8 then follows the path described above starting from the inlet 9of heat exchanger 2. This gaseous refrigerant stream 8 contains thelightest elements of the refrigerant stream of the refrigeration circuit7, i.e. typically nitrogen and methane.

“Temperature approximately equal to” another temperature means the sametemperature ±5° C.

The liquefied natural gas 6 resulting from the method according to thepresent invention may then, for example, be transferred to a storage ortransport device.

The method according to the present invention notably offers thefollowing advantages:

-   -   Energy optimization of the refrigeration cycle. In fact, the        liquid refrigerant streams are not subcooled more than is        necessary (typically characterized by correspondence between the        temperature of withdrawal from the exchanger at points 20 and        28), and the composition of the evaporated refrigerant stream        (having the lightest components) at the coldest outlet of the        main heat exchanger is improved.    -   Optimization of capital expenditure in particular by reducing        the size of the exchanger performing liquefaction of the        hydrocarbon-rich fraction, as a pump is not used in the        refrigeration circuit.

It will be understood that many additional changes in the details,materials, steps and arrangement of parts, which have been hereindescribed in order to explain the nature of the invention, may be madeby those skilled in the art within the principle and scope of theinvention as expressed in the appended claims. Thus, the presentinvention is not intended to be limited to the specific embodiments inthe examples given above.

1.-8. (canceled)
 9. A method for liquefying a hydrocarbon stream such asnatural gas starting from a feed stream comprising at least thefollowing steps: Step a): passing the feed gas against a mixedrefrigerant stream through a heat exchanger to supply an at leastpartially liquefied hydrocarbon stream having a temperature below −140°C.; Step b): withdrawing a mixed refrigerant stream from the heatexchanger from an outlet where the temperature in the heat exchanger ishighest; Step c): introducing the mixed refrigerant resulting from stepb) into a phase separating means in order to produce a gaseousrefrigerant stream and a first liquid refrigerant stream; Step d):passing the first liquid refrigerant stream resulting from step c) inthe heat exchanger starting from a first inlet and up to a so-calledintermediate outlet, beyond which the refrigerant stream thus obtainedis expanded, the temperature T1 at said outlet being such that saidexpansion produces a gas fraction below 20%; Step e): in parallel withstep d), compressing the gaseous refrigerant stream resulting from stepc) and then cooling before introducing the refrigerant stream thusobtained into a phase separating means in order to produce a gaseousrefrigerant stream and a second liquid refrigerant stream; Step f):passing the second liquid refrigerant stream resulting from step e) inthe heat exchanger starting from a second inlet and up to an outlet,beyond which the refrigerant stream thus obtained is expanded, thetemperature T2 at said outlet being above T1 and such that saidexpansion produces a gas fraction below 20%; Step g): passing thegaseous refrigerant stream resulting from step e) in the heat exchangerstarting from a third inlet and up to an outlet at a temperature T3, thelevel of which is the lowest of the temperature levels of said heatexchanger in order to produce a liquefied stream, and then expanding thestream thus obtained; Step h): passing the stream resulting from step g)in the heat exchanger from an inlet at a temperature T3 up to an outletat a temperature approximately equal to the temperature T2; Step i):mixing the refrigerant stream resulting from step h) with therefrigerant stream resulting from step f), then passing the mixture thusobtained in the heat exchanger from an inlet having a temperatureapproximately equal to T2 up to an outlet having a temperatureapproximately equal to T1; Step j): mixing the refrigerant streamresulting from step i) with the refrigerant stream resulting from stepd) and then passing the mixture thus obtained in the heat exchanger upto the outlet.
 10. The method as claimed in claim 9, wherein the mixedrefrigerant stream circulates in the closed-cycle refrigeration circuit.11. The method as claimed in claim 9, further comprising a step beforestep c) of compressing the mixed refrigerant resulting from step b)followed by cooling.
 12. The method as claimed in claim 9, wherein T1 isbetween −30° C. and −50° C.
 13. The method as claimed in claim 9,wherein T2 is between −80° C. and −110° C.
 14. The method as claimed inclaim 9, wherein T3 is between −140° C. and −170° C.
 15. The method asclaimed in claim 9, wherein the mixed refrigerant stream containsconstituents selected from the group consisting of nitrogen, methane,ethylene, ethane, butane and pentane.
 16. The method as claimed in claim9, wherein a pump is not used.